Frequency independent unidirectional antennas



D. E. ISBELL 3,210,767

FREQUENCY INDEPENDENT UNIDIRECTIONAL ANTENNAS Oct 5, 1965 2 Sheets-Sheet1 Filed May 3, 1960 XXX umxs Eumm MEQQQ W INVENTOR. Dwight E. lsbel/Merriam, Smif/z 8 Marsha/l ATTORNEYS FREQUENCY INDEPENDENTUNIDIRECTIONAL ANTENNAS Filed May 3. 1960 D. E. ISBELL Oct. 5, 1965 2Sheets-Sheet 2 INVENTOR. Dwighf E. lsbe/l Merriam, Smith 8 MarshallATTORNEYS United States Patent ()fi ice 3,210,767 Patented Oct. 5, 19653,210,767 FREQUENCY INDEPENDENT UNIDIRECTIONAL ANTENNAS Dwight E.Isbell, Seattle, Wash, assignor to The University of IllinoisFoundation, a non-profit corporation of Illinois Filed May 3, 1960, Ser.No. 26,589 15 Claims. (Cl. 343-7925) definite mathematical formula, eachof the dipoles being fed by a common feeder which introduces a phasereversal of 180 between connections to successive dipoles. The antennasof the invention provide unidirectional radiation patterns of constantbeamwidth and nearly constant input impedances over any desiredbandwidth.

The invention will be better understood from the following detaileddescription thereof taken in conjunction with the accompanying drawing,in which:

FIGURE 1 is a schematic plan view of an antenna made in accordance withthe principles of the invention;

FIGURE 2 is an isometric view of a practical antenna embodying theinvention; and

FIGURES 3 and 4 are radiation patterns of a typical antenna, in the Eplane and H plane, respectively.

Referring to FIGURE 1, it will be seen that the antenna of the inventionwas composed of a plurality of dipoles 10, 11, 12, etc., which arecoplanar and in parallel, side-by-side relationship. It will be notedthat the lengths of the successive dipoles and the spacing between thesedipoles is such that the ends of the dipoles fall on a pair of straightlines which intersect and form an angle 06. In the preferred embodimentthe antenna is symmetrical about a line passing through the midpoints ofthe dipoles, as shown.

The antenna is fed at its narrow end from a conventional source ofenergy, depicted in FIGURE 1 by alternator 13, by means of a balancedfeeder line consisting of conductors 14 and 16. It will be seen that thefeeder lines 14 and 16 are alternated between connections to consecutivedipoles, thereby producing a phase reversal between such connections.

The lengths of the dipoles and the spacing between dipoles are relatedby a constant scale factor 1- defined by the following equations:

T (n+l) (n+1) Ln ASH where 'r is a constant having a value less than 1,L is the length of any intermediate dipole in the array, L(n+1) is thelength of the adjacent smaller dipole, AS is the spacing between thedipole having the length L and the adjacent larger dipole, and A8 is thespacing between the dipole having the length L and the adjacent smallerdipole.

It will be seen from the geometry of the antennas, as given above, thatthe distance from the base line at the vertex of the angle cc to thedipoles forming the array are defined by the equation:

where X is the distance from the base line 0 to the dipole having thelength L X is the corresponding distance from the base line to theadjacent smaller dipole, and 1- has the significance previously given.

The radiation pattern of the antennas of the invention, having thegeometrical relationship among the several parts as defined above, isunidirectional in the negative X direction, i.e., extending to the leftfrom the narrow end of the antenna of FIGURE 1.

The construction of an actual antenna made in accordance with theinvention is shown in FIGURE 2. In this antenna the balanced lineconsists of two closelyspaced and parallel electrically conducting smalldiameter tubes 17 and 18 to which are attached the dipoles, each ofwhich consists of two individual dipole elements, e.g., 19 and 19a, 21and 21a, etc. It will be noted that each of the two elements making upone dipole is connected to a different one of said conductors 17 and 18,in a direction perpendicular to the plane determined by said conductors17 and 18. Moreover, considering either one of the conductors 1'7 and18, consecutive dipole elements along the length thereof extend inopposite directions. It will be seen that this construction has theeifect of alternating the phase of the connection between successivedipoles, as depicted schematically in FIGURE 1. Although the dipoles ofFIGURE 2 are not precisely coplanar, differing therefrom by the distancebetween the parallel conductors, in practice this distance is very smallso that the dipole elements are substantially coplanar and theadvantages of the invention are maintained. The antenna of FIGURE 2 maybe conveniently fed by means of a coaxial cable 22 positioned withinconductor 18, the central conductor 23 thereof extending to and makingelectrical connection with conductor 17 as shown.

As an example of the invention, an antenna of the type shown in FIGURE 2was constructed using 0.125 inch diameter tubing for the balanced lineand 0.050 inch diameter wire for the elements. The elements wereattached to the feeder line with soft solder, and the array was fed withminiature coaxial cable inserted through one of the balanced lineconductors. The antenna was defined by the parameters r=0.95 and 11:20".The antenna had a total of 15 dipoles, with the longest dipole elementbeing 2 /2" long, while the shortest element was one-half of thislength, or 1 The array was 7 /2 long.

Typical radiation patterns for the above-described antenna in the Eplane and the H plane are shown in FIGURES 3 and 4, respectively. Thesepatterns were found to remain essentially constant over the band ofabout 1100 to 1800 mc./sec. The minimum front-toback ratio over thisband was 17 db and the directivity over the range from about 1130 to1750 mc./sec. was better than 9 db over isotropic.

The performance of the above-described antenna clearly indicates thatthe antennas of the invention pro vide excellent rotatable beams for useparticularly in the HF to UHF spectrum. In comparison to the well-knownparasitic types of antennas which bear some resemblance to those of theinvention, such as the Yagi array, the antennas of the invention providea much wider bandwidth with essentially comparable directivity.Advantageously, however, the antennas of the invention need no adjustingfor their performance over a wide bandwidth, compared to the parasitictypes which must be adjusted by cut-and-try procedures for eachfrequency. Further experimental work with other antennas similar to thatdescribed above has indicated that the preferred values for theparameters which define the antennas of the invention include a range ofvalues for angle a be tween about 20 and with 1- having a value betweenabout 0.8 and about 0.95. When these parameters have values within thepreferred ranges the antennas were found to have essentially frequencyindependent performance over any desired bandwidth. The upper and lowerlimits of the bandwidths may be adjusted as desired by fixing thelengths of the longest dipole and the shortest dipole, respectively. Ithas been determined experimentally that the longest dipole elementshould be approximately 0.47 wavelength long at the lower limit and theshortest element should be about 0.38 wavelength long at the upperlimit. Moreover, in order to provide a suitable front-to-back ratio atthe low frequency limit, there should be at least 3 dipoles in the arrayand preferably about to 30 dipoles.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A broadband unidirectional antenna comprising an array ofsubstantially coplanar and parallel dipoles of progressively increasinglength and spacing in side-byside relationship, the ratio of the lengthsof any two adjacent dipoles being given by the formula where L is thelength of any intermediate dipole in the array, L n+1 is the length ofthe adjacent smaller dipole and 'r is a constant having a value lessthan 1, the spacing between said dipoles being given by the formulawhere AS is the spacing between the dipole having the length L and theadjacent larger dipole, AS is the spacing between the dipole having thelength L and the adjacent smaller dipole, and 1- has the significancepreviously assigned, said dipoles being fed in series by a common feederwhich alternates in phase between successive dipoles.

2. The array of claim 1 which is symmetrical about a line passingthrough the midpoint of each dipole in the array.

3. A broadband unidirectional antenna comprising an array of a pluralityof substantially coplanar and parallel dipoles of progressivelyincreasing length in side-by-side relationship, the ends of said dipolesfalling on a V-shaped line forming an angle a at its vertex, the ratioof the lengths of any pair of adjacent dipoles being given by theformula where L is the length of the longer dipole of the pair, L is thelength of the shorter dipole, and 7' is a constant having a value lessthan 1, the dipoles in said array being fed in series by a common feederwhich alternates 180 in phase between successive dipoles.

4. The antenna of claim 3 in which the angle 06 has a value betweenabout 20 and 100 and the constant T has a value between about 0.8 and0.95.

5. The antenna of claim 3 in which said feeder is a balanced line whichtwists 180 between the connections to successive dipoles.

6. A broadband unidirectional antenna comprising a balanced feeder lineconsisting of two closely spaced, straight and parallel conductors, aplurality of dipoles each consisting of two dipole elements, one ofwhich elements is connected to one of said conductors, the other elementbeing connected directly opposite the first to the other of saidconductors, the elements of any dipole extending in opposite directionsperpendicular to the plane determined by said conductors, consecutivedipole elements on each of said conductors extending in oppositedirections, the ratio of the lengths of the elements in any two adjacentdipoles being given by the formula where AS is the spacing between thedipole having the element length l and the adjacent larger dipole, AS isthe spacing between the dipole having the element length I and theadjacent smaller dipole, and 1- has the significance previouslyassigned.

7. The antenna of claim 6 wherein 7- has a value of about 0.8 to 0.95.

8. The antenna of claim 6 wherein said feeder line conductors aretubular.

9. An aerial system including at least one set of parallel dipolesspaced along and substantially perpendicular to the longitudinal axis ofa two-conductor balanced feeder to which the halves of the dipoles areconnected at their inner ends, said dipoles being of differentelectrical lengths increasing substantially logarithmically from theconnected end of the feeder to the other end and the dipole feederconnections being crossed over one another between adjacent dipoles, thespacings between which also increase substantially logarithmically fromsaid connected end to the other end.

10. An antenna system for wide-band use comprising a plurality ofsubstantially parallel conducting dipole elements arranged insubstantially collinear pairs, the opposite dipole elements of each pairconstituting dipole halves, a two-conductor balanced feeder having oneconductor connected to each of said elements at substantially the innerend thereof, each of said dipole halves in a pair being connected to adifferent feeder conductor, adjacent dipole elements being reverselyconnected to different conductors of the feeder, said dipole elementsbeing selectively spaced along and substantially perpendicular to saidfeeder, the elements of each pair being of substantially equal length,adjacent dipole elements of different pairs differing in length withrespect to each other by a substantially constant scale factor, theselective spacings between adjacent dipoles generally decreasing fromone end of the feeder to the other with the greatest spacing beingbetween the longest dipoles, and means to connect the feeder to anexternal circuit at substantially the location of the smallest of thedipole elements.

11. An antenna system for wide-band use comprising a plurality ofsubstantially parallel conducting dipole elements arranged insubstantially collinear pairs, the opposite dipole elements of each pairconstituting dipole halves, a two-conductor balanced feeder having oneconductor connected to each of said elements at substantially the innerend thereof, each of said dipole halves in a pair being connected to adifferent feeder conductor, adjacent dipole elements being reverselyconnected to different conductors of the feeder, said dipole elementsbeing selectively spaced along and substantially perpendicular to saidfeeder, the elements of each pair being of substantially equal length,adjacent dipole elements of different pairs differing in length withrespect to each other by a substantially constant scale factor, theselective spacings between the dipoles along the feeder differing fromeach other also by a substantially constant scale factor, the greatestspacing being between the longest dipoles, and means to connect thefeeder to an external circuit at substantially the location of thesmallest of the dipoles.

12. The aerial system of claim 11 in which said scale factors havevalues within the range from about 0.8 to about 0.95.

13. An antenna system for wide-band use comprising an array of at leastthree linear substantially parallel conducting dipoles, each dipolebeing composed of two opposite substantially collinear conductingelements, a two-conductor balanced feeder having one conductor connected to each of said elements at substantially the inner end thereof,adjacent parallel dipole elements being reversely connected to adifferent conductor of the feeder, the two elements of each dipole beingof substantially equal length and successive elements being of lengthswhich differ from one dipole to the next by a substantially constantscale factor within the range from about 0.8 to about 0.95, the dipolesbeing spaced from each other in a generally decreasing manner in thedirection of decreasing element length, and means to connect the feederconductors to an external circuit at substantially the location of thesmallest dipole elements.

14. An antenna system for wide-band use comprising a minimum of threepairs of linear substantially parallel conducting elements arrangedsubstantially coplanarly, each pair being substantially collinear andcomprising the halves of a dipole, a two-conductor feeder connected tothe inner ends of said collinear pairs of elements, adjacent parallelelements being connected to different conductors of the feeder so thatthe halves of the dipoles connect to different conductors of the feederand adjacent dipoles are reversely connected, the halves of each dipolebeing substantially the same length, adjacent dipole elements beingselectively spaced from each other along the feeder, the length of thesuccessive dipole elements along the feeder decreasing in accordancewith a substantially constant scale factor, each dipole and the feederbetween it and the adjacent dipole constituting a cell, the dimension ofthe several cells measured from the point of connection of one dipoleand the feeder to the outer end of the next smaller adjacent dipole alsodecreasing from one cell to the next in the direction of decreasingdipole length according to a substantially constant scale factor so thatthe combination of cells provides a substantially uniform wide-bandresponse, and means to connect an external circuit to the feederelements at substantially the location of the shortest of the dipoles.

15. An antenna system for wide-band use comprising a minimum of threepairs of substantially parallel and coplanar linear conducting elementsarranged in substantially collinear pairs, each pair of elementscomprising the halves of a dipole, a two-conductor feeder, one conductorof which is connected to each of said elements substantially at theinner end thereof, adjacent parallel elements being connected todifferent conductors of the feeder so that the halves of the dipolesconnect to different conductors of the feeder and adjacent dipoles arereversely connected, the halves of each dipole being substantially thesame length, adjacent dipole elements being selectively spaced from eachother along the feeder, the lengths of the elements decreasing from oneend of the feeder to the other substantially in accordance with asubstantially constant scale factor within the range from about 0.8 to0.95, each dipole and the feeder between it and the adjacent dipoleconstituting a cell, the cell dimension from the inner end of one dipoleto the outer end of the next smaller adjacent dipole also generallydecreasing from one cell to the next in the direction from the longer tothe shorter dipoles so that the combination of cells provides asubstantially uniform wide-band response, and means to connect anexternal circuit to the feeder elements at substantially the location ofthe shortest of the dipoles.

References Cited by the Examiner UNITED STATES PATENTS 2,192,532 3/40Katzin 343-811 2,507,225 5/50 Scheldorf 3438 14 X FOREIGN PATENTS1,023,498 1/5 8 Germany.

408,473 4/34 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner. GEORGE N. WESTBY, ELI LIEBERMAN,Examiners.

13. AN ANTENNA SYSTEM FOR WIDE-BAND USE COMPRISING AN ARRAY OF AT LEASTTHREE LINEAR SUBSTANTIALLY PARALLEL CONDUCTING DIPOLES, EACH DIPOLEBEING COMPOSED OF TWO OPPOSITE SUBSTANTIALLY COLLINEAR CONDUCTINGELEMENTS, A TWO-CONDUCTOR BALANCED FEEDER HAVING ONE CONDUCTOR CONNECTEDTO EACH OF SAID ELEMENTS AT SUBSTANTIALLY THE INNER END THEREOF,ADJACENT PARALLEL DIPOLE ELEMENTS BEING REVERSELY CONNECTED TO ADIFFERENT CONDUCTOR OF THE FEEDER, THE TWO ELEMENTS OF EACH DIPOLE BEINGOF SUBSTANTIALLY EQUAL LENGTH AND SUCCESSIVE ELEMENTS BEING OF LENGHTSWHICH DIFFER FROM ONE DIPOLE TO THE NEXT BY A SUBSTANTIALLY CONSTSANTSCALE FACTOR WITHIN THE RANGE FROM ABOUT 0.8 TO ABOUT 0.95, THE DIPOLESBEING SPACED FROM EACH OTHER IN A GENERALLY DECREASING MANNER IN THEDIRECTION OF DECREASING ELEMENT LENGTH, AND MEANS TO CONNECT THE FEEDERCONDUCTORS TO AN EXTERNAL CIRCUIT AT SUBSTANTIALLY THE LOCATION OF THESMALLEST DIPOLE ELEMENTS.